Astronomy Glossary

The following list gives definitions for many of the terms
used in this website and throughout any other astronomical literature you may
read. Definitions are listed in alphabetical order and many are linked
from other pages so you may quickly refer to an unfamiliar term from elsewhere
on the Guide to CCD Imaging website.

Clicking on the letters below will quickly link you to terms beginning
with that letter.

A/D Converter
The A/D Converter changes the analog signal from the CCD
camera into a digital signal which is read by the computer. How
finely the signal is divided digitally is determined by the bit
depth of the A/D Converter.

ADU
An ADU is an Analog/Digital Unit. This is the unit
of measure for the values in a CCD image. If you measure a value of
2500 ADU for a particular pixel in an image, you can calculate how many
electrons were actually collected by the corresponding photosite if you
know the system gain of the CCD, which determines the
conversion from electrons to ADUs.

Afocal Imaging
Afocal imaging involves taking a picture through both a
telescope and eyepiece. As opposed to prime focus
imaging, where the camera is attached directly to the telescope without
any eyepiece, afocal imaging requires an eyepiece. This is used with
CCD, DSLR, and film cameras for planetary imaging (called eyepiece
projection imaging). This is also the only way to image using
digicams which do not have removable lenses. The camera is attached
directly to the eyepiece.

Algorithm
In CCD imaging, an algorithm usually refers to a software
procedure, often for image processing. An algorithm is the
mathematical function which tells the computer how to modify the image.

Alt-Azimuth
Short for Altitude-Azimuth (also called Alt-Az).
Telescopes which are mounted so that they move up-down and left-right (as
opposed to equatorially) are called alt-az. This is a convenient
mounting configuration for visual observing as the eyepiece is always in a
convenient position. However, an equatorial wedge
must be used for photography or CCD imaging.

Anti-Blooming
This feature is added to a CCD chip to prevent pixel blooming.
This feature generally reduces sensitivity, well-depth, and
linear
response. For these reasons, non-anti-blooming chips are
popular, and there is even software available to
remove blooming streaks from CCD images.

Apparent Size
The apparent size of an object is how large the object
looks in the night sky. This measurement is called an angular size
since it measures how much of an angle in the sky the object subtends.
It is usually given in degrees, arcminutes, or arcseconds, depending on
the type of object. For example, the moon's apparent size is 1/2
degree. Jupiter's apparent size is around 45 arcseconds.

Array
Array refers to the actual layout of photosites
(or pixels) on a CCD chip. Chips are often
referred to by their array size, or number of pixels. For example
a KAF-1600 chip has an array that is 1530 pixels wide by 1024 pixels
high.

AstrometryAstrometry is the measurement of the position of an object, relative to
known fixed positions. For example, astrometric measurements of an
asteroid or comet as it moves can be used to determine the object's orbit.

Autoguider
An autoguider is a small CCD camera which is used to send
guiding corrections to the telescope's mount to increase tracking
accuracy. An autoguider is used in conjunction with a film camera
or CCD camera. Some CCD cameras have built-in autoguiders (see
self-guiding).

Binning
Binning involves combining pixels on
a CCD chip to create larger pixels. For example, taking a 2x2
square of pixels and creating one pixel that is twice the width and four
times the area of the original pixel. This is done to increase
sensitivity or to match a long focal length telescope to a CCD camera
with small pixels.

Bit Depth
A CCD camera takes an analog signal which must be
converted to a digital signal for the computer. This conversion is
done by the A/D Converter. Each pixel
has a value which is assigned a numerical value in the computer.
The bit depth determines how finely each pixel value can be divided,
meaning a greater bit depth can provide more information. A 12-bit
A/D converter divides the signal into 4096 levels of information.
A 16-bit converter provides 65,536 levels.

Blooming
Each photosite of a CCD chip can
contain a certain amount of electric charge. This amount is
determined by the well depth of the CCD. When
the well-depth is exceeded, electric charge "bleeds" out of
the photosite appearing in an image as a bright streak extending
vertically from a bright source in the image (usually a star).
This effect can be minimized or eliminated by using a CCD with an
anti-blooming
gate. Anti-blooming filters are available for image processing
software to remove blooming streaks from images.

CCDCCD stands for Charge-Coupled Device. CCDs are
silicon chips with receptors (photosites) to
collect light. The light is turned into electric charge and read
out to a computer where an image is displayed. Astronomical CCD
cameras, digital cameras, and digital camcorders are all based on this
same charge-coupled device.

Charge Transfer EfficiencyPixels are read out one at a time into a register
along one side of the CCD chip. The charge collected in each pixel
is shifted down the CCD chip toward the register. Some charge is
lost during this transfer. The ability of a CCD camera to keep as
much charge as possible from being lost during readout is known as its
Charge Transfer Efficiency.

Chromatic AberrationThis aberration is found in telescope systems with lenses, but is
generally associated specifically with refractors where it is most
noticeable. In a refractor, light passes through a lens and is bent
to reach a focus point. Each wavelength of light is bent
differently, so they do not all meet at the same point of focus. The
result is an out-of-focus glow, usually purple in color since the violet
light is least likely to meet focus with the other colors. Some
refractors are specially corrected for this aberration. CCD cameras
are far more sensitive to ultraviolet and infrared light than the human
eye, so only the very best refractors are suitable for high-resolution CCD
imaging.

CMOS
CMOS stands for Complimentary Metal Oxide Semiconductor.
CMOS chips are similar to CCD chips but have some differences in how they
function. In a CCD, the charge from each pixel is read out to an
output node where it is converted to a voltage. This voltage is then
sent off chip to be digitized and sent to the computer. In a CMOS
chip, each pixel outputs a voltage so there is no need to convert off
chip. Digitization is done on chip, allowing for a less complex
overall system (though the chip itself is more complex). CMOS
sensors are used in some digital cameras and offer very low noise
characteristics (although this is not necessarily an inherent
characteristic of all CMOS chips).

Color Filter Wheel
Many CCD cameras are inherently black and white. In
order to get a color image from such a camera, three images must be taken
through red, green, and blue filters and then combined into a single color
image. (This is called tri-color imaging.) A color filter
wheel holds the color filters (and sometimes other optional filters) and
is normally motorized to automate the process of selecting a filter.

Coma
Coma is an aberration seen in certain telescope designs.
It is an off-axis aberration, meaning it is not seen in the center of the
field but grows progressively worse toward the edge of the field.
Newtonians and commercial Schmidt-Cassegrain telescopes suffer from coma,
although it is most noticeable in Newtonian scopes. Coma correctors
can be used to eliminate this aberration. Achromatic refractors also
tend to suffer from some coma, although apochromatic refractors usually
eliminate or minimize the aberration. Ritchey-Chrétien designs are
specifically made to eliminate coma.

CMYK
CMYK imaging is an alternative form of color imaging. Usually
RGB or LRGB imaging is used, but
CMY (cyan, magenta, and yellow) filters can be substituted for RGB
filters, and the L (luminance) layer of and LRGB becomes the K (or black)
layer in a CMYK image.

Dark CurrentAstronomical CCDs are so sensitive that they can detect
electronic noise generated by the CCD chip itself. The amount of
noise created by the CCD chip is known as Dark Current. Dark
current is a function of temperature and cooling the CCD chip can remove
much of the noise. However, not all noise is eliminated and so
imagers rely on Dark Frames to remove much of the
remaining noise.

Dark Frame
A dark frame is simply a CCD image taken with the camera's
shutter closed. This image detects noise generated by dark
current. An image of the same duration is taken with the
shutter open, detecting both the noise and the object being
imaged. Using image processing software to subtract the dark frame
from the raw image leaves only the object. The dark noise is
eliminated.

DDP
DDP stands for Digital Development Processing. This
an image processing routine used in MaxIm DL software. DDP
processing allows both bright and dim parts of an astronomical object to
be displayed at the same time. DDP essentially compresses
washed-out regions of an object into a range that the computer can
display. This process is especially useful on galaxies which have
bright cores and faint spiral arms.

Download Speed
Also called Transfer Rate, or System Throughput, this
describes how quickly the information in the CCD is transferred to the
computer. There are several factors involved, including reading the
data from the CCD chip, sending it to the computer, and then displaying it
on the screen. The total download time is how long you wait from the
end of the exposure until you see the image. For large-format CCDs
using parallel-port interfaces this might be as much as 2 minutes, while
for smaller chips with high-speed USB interfaces there might be less than
1 second of download time.

Dynamic Range
Dynamic range describes the range of brightness that can
be seen by a CCD camera. CCD cameras have a much greater dynamic
range than does film. A CCD's ability to capture both faint and
bright details in determined by its dynamic range.

FastarFastar was Celestron's high-speed CCD imaging system.
Fastar involves removing a Schmidt-Cassegrain telescope's secondary mirror
and placing the CCD camera at the front of the telescope. This
provides a wide field of view and a vary fast imaging system. A
telescope which is said to be "Fastar compatible" has a removable
secondary mirror. Celestron has discontinued Fastar.
HyperStar is Starizona's improved version of the
Fastar lens.

FFT Filters
FFT stands for Fast Fourier Transform. FFT filters
are image processing algorithms applied to CCD images, usually to
sharpen or smooth and image. FFT filters have the same function as
kernel filters, but are applied differently so the
processing is faster. Applying an FFT to an image changes the
image so that applying a kernel filter is easier. The FFT function
is then reversed and a filtered image results.

Filter Wheel
Black and white CCD cameras use color filters to separate
the three primary colors (red, green, and blue) in order to produce full
color images. Usually an automated mechanical wheel holds the
filters in front of the camera and rotates the appropriate filter into
place before an exposure. Filter wheels can also hold narrowband
or photometric filters for different types of imaging.

Flat Field
Out-of-focus dust particles in the optical system and vignetting
can lead to an unevenly illuminated CCD chip. By taking an image
of an evenly illuminated source (such as the evening sky after sunset or
the inside of an observatory dome) the differences in illumination
across the field of view can be removed using image processing software.

Focal Length
This is the effective length of the path which the light
must follow through a telescope. In a refractor or Newtonian design
the focal length is equal to the actual length of the light path from the
primary mirror to the focus point (where a CCD would be placed). In
a Cassegrain type of telescope, the effective focal length is usually
about 5 times longer than the actual length of the light path due to the
magnifying effect of the secondary mirror.

Focal Ratio
This is the ratio between the focal length of a telescope
and the aperture. A telescope with an 8" aperture and 80" (2000mm)
focal length has a focal ratio of f/10. Smaller focal ratios equate
to shorter exposure times. An f/4 system is faster than an f/6
system, for example.

Focal Reducer
A focal reducer is used to decrease the focal ratio (and
focal length) of a telescope. A smaller focal ratio yields a faster
optical system and thus a shorter exposure time. The trade off is
reduced image scale, but this is usually acceptable in return for a wider
field of view and shorter exposure. Focal reducers typically attach
to the back of the telescope, just ahead of the camera in the optical
path. They are made in a variety of focal reduction factors, from
0.18x to 0.8x. Popular focal reducers for SCTs are 0.33x and 0.63x
reducers, often called f/3.3 and f/6.3 reducers since most SCTs have an
inherent focal ratio of f/10 and thus the reducers yield these new focal
ratios. A focal reducer is a highly recommended accessory for
imaging.

Full Well Capacity
Each photosite on a CCD chip can
contain a certain number of electrons. This number is called the
Full Well Capacity. Within a certain chip full well capacity is a
function of binning, so that binning 2 times, for
example, quadruples the full well capacity. Of course, 2x binning
also makes the CCD effectively 4 times more sensitive and so the wells
will fill in the same amount of time. Chips with
anti-blooming
generally have lower full well capacity, but will not bloom
when the well is filled.

GainThe system gain of a CCD camera tells you how many
electrons are represented by each ADU. Gain is
expressed in electrons/ADU. For example, a CCD camera with a gain of
1.3 converts 1.3 electrons into one ADU. So, if you were to measure
a pixel value of 2500 ADU for some area of the image, that value
corresponds to 2500 x 1.3 = 3250 electrons collected by that particular
photosite. Equivalently, a camera with a gain of 2.5 would convert
those same 3250 electrons into an ADU value of just 1300. This can
be useful in calculations of optimum exposure times and
photometric
measurements. (Technically, most cameras add a 100 ADU
pedestal, so ADU values are normally 100 less than
measured.)

Guiding
No telescope mount can track perfectly, yet for CCD
imaging it is necessary to very accurately track the object being
imaged. This is done by guiding on a star to make small
corrections to the mount to accurately follow the star. This makes
up for any errors in the telescope's drive system. Guiding can be
done manually by watching a star through a crosshair eyepiece, or, more
commonly, by using an autoguider to automatically guide. See also,
Unguided Exposure, Autoguider,
Self-Guiding, and Track &
Accumulate.

HistogramA histogram is a graph of number of pixels versus pixel
value. Pixel values run from lowest (displayed as black) to
highest (displayed as white). A bar is plotted for each pixel
value showing the number of pixels in the image with that value.
An astronomical image typically has more bars toward the lower (darker)
end of the histogram since most astroimages contain are large amount of
dark sky around a brighter (but small) object. For more
information on histograms and how they are used for image processing and
display, click here.

HyperStarHyperStar is Starizona's improved version of Celestron's
Fastar lens. Fastar optics allows the secondary mirror to be removed
from an SCT and replaced with a lens assembly that allows fast f/1.8
imaging. The HyperStar lens has better optical correction than the
original Fastar and is compatible with many more CCD cameras. It is
available for Celestron's 8", 11" and 14" Schmidt-Cassegrain telescopes.

Image ScaleImage scale is basically equivalent to magnification.
Image scale is related to focal length in that the longer the focal length
of a telescope, the greater the image scale will be. With film
systems, image scale was typically measured in arcseconds per millimeter.
But since CCDs have pixel sizes measured in microns, image scale is
usually measured now in arcseconds per micron. This can be used to
determine how much sky is covered by a given pixel in a CCD camera (which
gives the resolution), or how large an object might appear in an image.
The formula for image scale = 206 / Focal Length. This gives the
result in arcsec/micron. A related equation gives resolution in
arcsec/pixel. This formula = (Pixel Size * 206) / Focal Length.
Focal length in these equations is in millimeters.

ISO
ISO was originally a measure of film speed. This
has been adopted for digital camera use as well. In a film camera,
changing the film was required if a change in film speed (sensitivity) was
required. Typical ISO film speeds are 50, 100, 200, 400, and 800.
Each correlates to a 2x improvement in speed over the previous ISO.
Digital cameras have ISOs that are comparable to film speeds but can be
changed electronically. Just like film, increasing the ISO of a
digital camera increases noise (analogous to the grain in high-speed
film). High ISOs are preferable for astronomical imaging, but many
small digital cameras have high noise levels associated with high ISOs.
Digital SLRs, on the other hand, tend to have low noise at high ISOs,
making them suitable for astronomical imaging.

Kernel FiltersKernel filters are image processing algorithms applied to
a CCD image, generally to smooth or sharpen an image. A kernel is
a small grid which tells the computer how to change the value of a
certain pixel based on the values of neighboring pixels. The most
common types of kernel filters are low-pass ("smoothing" and
"blurring" filters) and high-pass ("sharpening"
filters). See also FFT filters.

Linear ResponseA CCD camera with a "linear response" has
sensitivity such that doubling the exposure time of an object of a
certain brightness will result in an image twice as bright. There
is a linear relationship between exposure time and brightness.
This is especially useful for making magnitude measurements of variable
stars, comets, asteroids, or supernovae. A camera with a nonlinear
response is not suitable for making magnitude estimates since a star
which appears twice as bright in an image is not necessarily twice as
bright in actuality. CCD cameras equipped with an
anti-blooming
feature are generally nonlinear. For taking pretty pictures linear
response makes no difference.

LRGB
Images taken with a black and white CCD camera through
red, green, and blue filters are combined to make RGB
images. An interesting effect of human vision is that we get most
of the spatial information about an image from the brightness, or
luminance, portion of the image and not from the color, or hue,
portion. This means it is possible to take a low resolution color
image and combine it with a high resolution black and white image to
make an LRGB image (the L standing for luminance). This is a
definite advantage for CCD imaging; since placing a filter in
front of a camera decreases its sensitivity, color images can be binned
to gain higher sensitivity at the expense of resolution. As long
as the low-resolution color image is combined with a high-res luminance
image (taken unfiltered and thus at the camera's maximum sensitivity)
the full resolution is maintained and the total exposure time is
decreased.

Maximum Entropy DeconvolutionThis is an image processing routine found in the MaxIm DL
software package. It is used to enhance detail in CCD images that
are slightly blurred due to atmospheric effects, tracking errors, or
imperfect optics. The routine works by attempting to match an ideal
image (essentially a perfect star image) to the actual blurred image.
The image is adjusted through a series of iterations to a closer
approximation of an ideal image.

Median Combine
When combining images there are two methods
employed. Most often images are summed to
get as much information as possible. However, noisy images can be
smoothed to some extent by taking the median value of all the pixels
in the images. Median combining requires that there be at least 3 images.
(2 images can be summed or averaged.)

Monochrome Imaging
A monochrome camera images in black and white. To create color images with
this type of camera, red, green, and blue filters are used. An image
through a given filter still appears in black and white, but when all three
filtered images are combined the result is a full color image. One-shot
color cameras differ in taking a single picture that is already full color.

Mosaic
CCD chips are usually small, and many deep-sky
astronomical objects are big. Many CCDs, especially if used with
long focal length telescopes, will not be able to fit a large object
into the field of view. One solution is to make a mosaic using
multiple images of smaller section of the object. Slightly
overlapping images are taken of sections of the object and are combined
in the computer. For more information on creating mosaics,click
here.

Narrowband ImagingNarrowband imaging uses filters to isolate specific
wavelengths of light. This allows very detailed images to be
obtained, especially of nebulas. Using narrowband filters isolates
certain gases, showing details not normally visible in regular RGB color
images. Narrowband filters also allow imaging in light-polluted
skies or during moonlit times of the month. Exposure times are
increased with narrowband filters. Typical filters are
Hydrogen-Alpha, Oxygen-III, and Sulfur-II.

One-Shot Color ImagingNormal black-and-white CCD cameras require a color filter
wheel with red, green, and blue filters in order to obtain color images.
One-shot color cameras, on the other hand, do not need separate color
filters. They can capture a color image in a single exposure.
Typically, one-shot color cameras are less sensitive than non-color
cameras, but since they require 1/3 the number of exposures to capture a
color image, the end result is often a shorter amount of time spent
imaging.

PedestalA pedestal is a value added by image processing software
to the pixel values of an image to avoid zero values from appearing in
pixel math calculations. Normally a value of 100 ADU
is added to the standard pixel values. The prevents random noise
from lowering a value to zero and affecting pixel math calculations.
The pedestal means that if you measure a pixel value to be 2500 ADU, it is
really 2400 ADU.

Periodic Error Correction (PEC)
All telescope mount drive gears suffer from periodic
error. No gear can be machined perfectly, although some are
certainly built with more precision than others (which is why some mounts
cost $300 and some cost $10,000). Since the drive gear repeats its
inherent errors at a given interval (however long the gear takes to
revolve once, usually 5-10 minutes), the errors are repeated and, in
theory, removable. PEC is built into certain mount electronics to
record drive errors and repeat corrections to those errors. PEC
allows longer unguided exposures. With guiding, PEC is usually not
necessary since corrections are being made almost constantly.

Piggyback
Often small telescopes or camera lenses are piggybacked,
or mounted on top of another telescope. Camera lenses and small
refractors are popular for wide-field imaging and astronomers who own a
larger scope such as an SCT will often mount the smaller instrument on top
of the larger, using the big scope as a tracking platform. This
eliminates the need for a separate mount for the smaller instrument and
allows the larger scope to be used for guiding.

Pixel
Pixel is short for "picture element", and describes the tiny
squares which make up a CCD image. Pixels can refer either to the
individual squares in an image or the actual light-sensitive squares on a
CCD chip (also called photosites). The
number of pixels in an image is often called the resolution, although
resolution also refers to the physical size of the pixels (photosites) on
a CCD chip.

PhotometryPhotometric measurements are used to determine the brightness of an object.
This data is used to analyze variable stars, comets, asteroids, and more.

Photosite
Photosites are the individual light-sensitive squares (or
rectangles) of a CCD chip. A CCD chip is comprised of thousands or
millions of photosites. In a CCD image, usually each photosites
becomes an individual pixel in the final image
displayed on screen, but sometimes the photosites are
binned and 4 or 9 individual photosites work effectively as one larger
photosite to create a single pixel. Photosites are almost always
called pixels, except when a distinction needs to be made between
photosites and image pixels.

Prime Focus
Prime focus imaging involves attaching a camera directly
to a telescope without any intermediate eyepiece or camera lens.
This is as opposed to afocal imaging which uses an
eyepiece or eyepiece and camera lens to increase the magnification (such
as for planetary imaging). Prime focus imaging is the normal setup
used for deep-sky astrophotography as it allows wider fields of view and
faster focal ratios.

Quantum EfficiencyAs photons of light hit a CCD chip they are converted into
electrons which are stored and then read out at the end of the
exposure. But not every photon that hits the chip is converted
into an electron. How many photons are converted depends on the
camera's quantum efficiency, or QE. QE is expressed as a
percentage of the number of photons converted. If all the photons
produced electrons, the QE would be 100%. Most amateur CCD cameras
have QEs in the range of 25-50%. More advanced
"back-illuminated" CCDs have QEs around 85%. Supercooled
professional CCDs used at observatories have QEs closer to 98%.
Compare this to film, which has a typical QE of around 2%. You can
see why CCDs are so much faster!

Readout NoiseThis is electronic noise generated in the CCD camera when
the data is read out from the camera to the computer.

Resolution
There are two common definitions for resolution. One
refers to the level of detail a CCD camera can capture, usually expressed
in arcseconds per pixel. Smaller pixels produce
a higher resolution, meaning smaller details can be seen. Resolution
is also taken to mean the total number of pixels in a CCD chip or in a CCD
image. Often this is given as width and height in pixels, similar to
computer screen resolution. For example, a CCD chip might have a
resolution of 1600x1200 pixels.

RGB
RGB stands for Red, Green, Blue. Most CCDs are
inherently black and white and require a color
filter wheel to take color images. When red, green, and blue
filters are used to create a color image, it is termed RGB imaging and the
resulting picture is an RGB image. This is the most common form of
color imaging, but there are other techniques used as well, such as
LRGB or CMYK imaging.

SamplingSampling refers to the proper matching of
pixel size to focal length to achieve the
best possible resolution. Sampling is the
essentially number of pixels that a star image covers. If too few,
the image is undersampled and does not achieve the best possible
resolution. If too many, the image is oversampled which
unnecessarily reduces efficiency.

Scaling
Scaling is a way of adjusting an image to bring out more
details. Scaling works by altering an image's histogram, usually to
compress the range between bright and faint portions of the image so that
both can be displayed by the limited range of a computer monitor.

Seeing
Seeing conditions refer to the steadiness of the
atmosphere. Seeing is critical for obtaining sharp images and is
especially critical for planetary imaging. In fact, it is the single
more important factor in capturing good planetary images. Seeing is
often confused with transparency.
Transparency is the clarity of the atmosphere, but has nothing to
do with the steadiness of the air. In fact, the two are generally
mutually exclusive. Nights of good transparency (dark, clear nights)
often have poor seeing. And the steadiest skies often come on hazy
nights of lousy transparency.

Self-Guiding
Self-Guiding is a feature of certain SBIG CCD cameras.
In these cameras there are two CCD chips: one for imaging, and a
smaller one for guiding the telescope. During long exposures, errors
in a telescope's drive prevent perfect star images. Guiding
eliminates this problem, but for most CCD cameras a second separate CCD
must be used to guide. Self-Guiding CCDs simply use their built-in
guiding chips to eliminate the need for a second camera. Also, a
guidescope or off-axis guider does not need to be used, avoiding some of
the other problems inherent in guiding a telescope using these methods.

Sharpening
Sharpening is a software routine used to enhance detail in
a CCD image. There are numerous methods employed for sharpening
images--there are even whole programs dedicated only to sharpening.
Some are simple sharpening routines, while others involve more complex
algorithms (such as the unsharp mask technique), and some employ iterative
routines to produce successively sharper images while attempting to
minimize the noise inherently generated in sharpening (the Lucy-Richardson
and similar techniques).

Spherical Aberration
This aberration comes from the fact that a spherical lens
or mirror cannot focus all the light rays from a star to a single point.
A proper combination of spherical optics or an aspheric optic will
minimize this aberration. The corrector lens on the front of a
Schmidt-Cassegrain, for example, is an aspheric lens which corrects the
aberration from the spherical primary mirror.

StackingCombining, or stacking, images is a way to enhance the image by reducing
noise. Multiple exposures of the same object are combined when stacking
images. Since the subject in each image is the same, but noise in the
image changes randomly from one picture to the next, stacking the frames
together decreases the overall amount of noise. Mathematically, exposures
can be stacked by summing or averaging the images, or by more complex
algorithms. Stacking is done automatically using
image processing software.

Summing Images
Since astronomical objects tend to be faint, it is often
desirable to add CCD images together to increase the detail visible in the
image. This process is known as summing. Summing images will
also, however, increase the noise in the image.

System Throughput
Also called Transfer Rate, or Download Speed, this
describes how quickly the information in the CCD is transferred to the
computer. There are several factors involved, including reading the
data from the CCD chip, sending it to the computer, and then displaying it
on the screen. The total download time is how long you wait from the
end of the exposure until you see the image. For older large-format CCDs
using parallel-port interfaces this might be as much as 3 minutes, while
for newer cameras with high-speed USB 2.0 interfaces there might be less than
1 second of download time.

Track & AccumulateSBIG's patented system for taking the equivalent of a long
exposure without having to guide the telescope. Instead of guiding
during an exposure (which requires either a second CCD camera or
Self-Guiding camera), corrections are made between
each of a series of exposures which are then combined into a single longer
exposure. By keeping the individual exposures short enough, tracking
errors do not interfere. However, the individual exposures must be
kept shorter than a certain limit imposed by the mount's inherent tracking
ability. This is typically 30-60 seconds for most mounts.

Transparency
Transparency refers to the clarity of the
atmosphere. A night of high transparency is very dark and clear.
This is ideal for deep sky imaging and observing. Transparency is
often confused with seeing. Seeing conditions
refer to the steadiness of the atmosphere, but has nothing to do
with the clarity of the air. In fact, the two are generally mutually
exclusive. Nights of good transparency (dark, clear nights) often
have poor seeing. And the steadiest skies often come on hazy nights
of lousy transparency.

Tri-Color Imaging
Most astronomical CCD cameras are inherently
black-and-white. To take color images, red, green, and blue filters
are placed in front of the camera to isolate each color. Each image
is then combined later using image processing software to create a color
image. This process is called tri-color imaging. See also,
RGB, LRGB, and CMYK.

Unguided ExposureThere are small tracking errors inherent in every
telescope system. To avoid these errors ruining the exposure there
are two methods that are used. One is to guide the telescope using
either a second CCD camera or by using a Self-Guiding
CCD. The other way to do it is to take an unguided exposure, short
enough to keep the tracking errors from being seen. It is possible
to combine multiple unguided exposures to create the effect of a single
guided exposure. Often high-quality telescope mounts give tracking
accuracy specifications in terms of how long an unguided exposure they are
capable of taking.

Unsharp Mask
Unsharp mask is an image-processing technique used to
sharpen an image and enhance detail. True unsharp masking is done by
subtracting a blurred version of an image from the original. Unsharp
mask routines in software simulate this effect and make it easier to have
control over the final image.

VignettingVignetting is a darkening of the corners of an image,
usually caused by light falling off toward the edge of the CCD chip due to
the optical system. Typically vignetting is seen most prominently in
fast-focal-ratio and wide-angle systems such as camera lenses.
Vignetting is usually removed by using a flat field.

WedgeAn equatorial wedge is used for imaging with a
fork-mounted telescope. Most computerized telescopes have
alt-azimuth fork mounts where the fork point straight up and down.
This configuration is easiest to use because the eyepiece is always in a
convenient position and the telescope is less bulky. However, field
rotation results if the telescope is not polar aligned. An
equatorial wedge mounts between the telescope and tripod to allow the
forks to be aimed toward Polaris so the telescope can track in just one
axis, eliminating field rotation.

Well Depth
Well depth is a measure of how much charge an individual
photosite on a CCD chip can contain. Well depth is generally
measured in electrons. For example, a Kodak KAF-1300 CCD chip has a
well-depth of 150,000 electrons. When more charge than this fills
the photosite, blooming occurs unless the CCD chip
has a built-in anti-blooming protection. In
general, anti-blooming chips have much lower well depths, which is one of
the reason non-anti-blooming CCDs are popular (they are much more
sensitive).